Seat Belt Control Module: Description and Operation

Vehicles up to 09/87:
The control unit, also referred to as the triggering unit or crash sensor, is responsible for determining the severity of a collision and, if necessary, triggering the restraint system at the correct time.

General:

Two integrated circuits are accommodated together with an acceleration sensor in a common housing.

The housing and the electrical connections have been designed in such a way as to virtually exclude any malfunctions due to electrical interference in the vehicle electrical system caused by the switching of various electrical consumer components on and off, and by electromagnetic interference on public highways.

Function

The electronic control unit processes the deceleration of the vehicle during a frontal collision and must therefore be rigidly coupled with the vehicle.

Incorrect triggerings are avoided by means of an integral mercury switch which interrupts the ignition circuits of the airbag and belt tensioner restraint system during normal driving, and by a predetermined deceleration threshold (as).

The acceleration sensor (1) in the control unit records the deceleration or acceleration acting on the vehicle on the longitudinal direction. and generates a signal. This signal is sent via the high-pass filter (2) to the amplifier (3) which adapts the signal strength to the following processing stages. The acceleration signal is restricted in the limiter (4) and is reduced by a constant acceleration threshold (as) in a subtractor circuit (5).

The integrator (6) then integrates the signal produced and passes it on to the trigger switch (7) for evaluation. If the fixed triggering speed V is exceeded, the output stages (20) are activated. This effects triggering of the driver's airbag and the belt tensioners by means of the detonators (19) when the test cycle is complete. An additional precaution against incorrect triggering is a mercury switch (18) installed in the control unit. The control unit also contains a further circuit to monitor the ignition circuits and to perform its own function check. Malfunctions are indicated by the warning lamp (21).

1 Acceleration sensor
2 High-pass filter
3 Amplifier
4 Limiter
5 Subtractor circuit
6 Integrator
7 Trigger switch
8 Voltage monitoring
9 Switch-on delay
10 Logic circuit
11 Ignition circuit monitoring
12 Fault memory
13 Voltage transformer
14 Reserve power source
15 Reserve power source monitoring
16 Sequence control system
17 Output stage monitoring
18 Mercury switch
19 Detonators for aibag and belt tensioner generators
20 Output stages
21 Warning lamp
22 Test switch

Vehicles From 09/87 or 01/88:

12-pin Control Unit, Driver Airbag:

As of 09/87 or 01/88, the airbag restraint system has a new control unit with integral reserve power source and voltage transformer. This means that the reserve power source and voltage transformer previously accommodated in separate housings are no longer required.

Function:
The control unit processes the vehicle deceleration during a frontal collision and is rigidly coupled on a support bracket with the transmission tunnel of the vehicle (as before). By means of an integral safety switch which interrupts the ignition circuits for the driver airbag and belt tensioner restraint system during normal driving, the airbag and belt tensioners can only be triggered by an actual acceleration. This prevents any malfunctions.

An acceleration sensor installed in the control unit detects the deceleration acting on the vehicle in the longitudinal direction and sends an appropriately conditioned signal to the trigger switches for evaluation. If the fixed triggering thresholds of both trigger switches are exceeded, the output stages are activated as is appropriate for the circumstances detected via the belt buckle switch (buckle latch in/not in belt buckle) This results in triggering of the system (see table below). If the power supply is interrupted during a collision, the driver airbag can still be triggered within 100 ms, but not the two belt tensioners. The warning lamp lights up if the vehicle voltage is < 9.5 V for longer than 10 s. The lamp goes out at a voltage > 10 V.

The housing and the electrical connections have been designed in such a way as to virtually exclude malfunctions due to electrical interference in the vehicle electrical system caused by the switching of electric consumer units on and off, and by electromagnetic radiation on public highways.

Belt Buckle, Except Coupe:

Belt Buckle, Coupe:

In order to detect whether the buckle latches are inserted in the belt buckles or not, new belt buckles with separate switches and resistors were necessary. This results in the triggering logic listed below.

Release Logic, Driver's Airbag and Belt Tensioner:

Explanation:
In a minor accident, in which the safety belt provides adequate protection, only the belt tensioner is activated when the first release threshold is passed, provided the belt buckle is fastened.

In a more serious accident, whereby the second threshold is passed, the airbag is also activated.

Release Logic, Driver's and Front Pass. Airbag and Belt Tensioner:

Explanation:
In a minor accident, in which the safety belt provides adequate protection, only the belt tensioner is acuvated when the first release threshold is passed, provided the belt buckle is fastened.

If the belt buckles are not fastened or the second release threshold is passed, the driver's and passenger's airbags will be activated instead of, or in addition to, the belt tensioners.